“High Brightness and High Polarization Electron Source

for Electron Microscope”

Tsutomu NAKANISHI(Department of Physics, Graduate School of Science,

Nagoya University)

PESP-2008 Workshop (3. October, 2008 @J-Lab)

Beam requirements for electron sources from

three kinds of electron accelerators

Φ10mm

Φ1mm

Φ1μm

High peak current（≥ 10A ）

High average current（≥ 10mA ）

High current density（≥ 1A/mm2 ）

ILC

ERL

SPLEEM

Contents of this talkhave already explained

partially in

1) Talk by Toru Ujihara,[ R/D of transmission PC ]

2) Poster by Naoto Yamamoto (“small” Y)[ Proto-type SPLEEM gun

performances ]

Specimen

Objective lens

Beam separator

LaB6 emitter

Energy analyzer（ 90°or 180°bend ）

Screen

Objective lens

Image lens Focus lens

Contrast aperture

h

PEEM

LEEM

LEEM / PEEM type Electron Microscope(developed by E. Bauer)

エネルギー分析器

試料準備室

CCD カメラ

マニピュレーター

高輝度水銀ランプシステム

電子銃

コントラストアパーチャー

入射側アパーチャー（イルミネーション　　　アパーチャー）

制限視野アパーチャー

電子ビーム

測定室

LaB6 Gun

Koshikawa & Yasue Group

Made by ELMITEC co. (Germany)

FOV10m0-3.2MLbcc

Dynamic observation of Cu thin film growth on W(110) at 100 ℃

layer-by-layer growth(Room temp. - ~ 150℃)

The third layer does not start just after

2.13MLbcc and it start at around 2.47

MLbcc.Coverage[MLbcc]1 2 30 2.13 2.47

Polarization of incident beam

Imaging of magnetic domainsImaging of magnetic domains

LEEM Image

Mag. Domain Image

:P

:Magnetization of surfaceM

P

M

A : Magnetic contrast (Asymmetry)

2

III

IIIIA

P

P

I+ = I + I = I

Mag. Domain contrast : LEEM contrast : I +

MP A

Pure Spin effects can be obtained in Magnetic domain images

Proposal of this work (April 2008)

Real-time observation of

magnetic domain formation process

Approved at September 2005 by Japan Science and Technology Agency (JST), as Technology Development Program for Advanced Measurement and Analysis (Program-T)

talk contents

1. Procedure for Higher Brightness Transmission Type Photocathode

2. A 20keV Test-Gun Apparatus Performances

Beam Performances

3. A 20keV gun for SPLEEMAssembling finished

Beam test in Progress

　 HV= － 20kV 、 gap-width=5.34mm Needle-tip　(20nm　radius)　

NEA-GaAs　emitter

M.Kuwahara et al. JJAP 45 (2006) 6245● 　 Field emitter polarized electron source

▲　 Serious Problem : Current limit　 ( Tip melt-down by self-heating ）　　（ current / tip 30 nA ）

Our first trial toward higher brightness （ 2003 ）

This work toward much higher brightness (2005)

Laser

Photocathode

Anode

Electron

Lens f=a few mm

Photocathode Anode

Mirror

Spherical Condenser

Lens f >20cm

Lens f >20cm

Laser

Laser

Electron

Electron

▲ 　 Conventional TypeLaser spot size 50m

● 　 New transmission type　　 Laser spot size

≈ diffraction limit a few m

Laser spot size (exp.)1.3μm(FWHM) @λ ： 777nm)

Advantage : Electron & Laser beam lines do not interfere

Lens stage to make the minimum laser spot

CCDcamera

Ti-SapphireLaser

Fiber Collimater

Polarizing Beam splitter

Quarter WaveplateImaging Lens

Positioner

XHV

PhotocathodeFocusing Lens

Electron Beam

Optical Fiber

FocusingLens

Photo-cathode 0

0.25

0.5

0.75

1

-3 -2 -1 0 1 2 3

Laser Spot Profile

YX

Inte

nsity

[A.U

.]

Posision [m]

FMHW Y : 1.4 mX : 1.3 m

talk contents

1. Procedure for Higher Brightness Transmission Type Photocathode

2. A 20keV Test-Gun Apparatus Performances

Beam Performances

3. A 20keV gun for SPLEEMAssembling finished

Beam test in Progress

A 20keV test-gun’s Compositions

NEA activation chamber

Gun chamber

Beam size monitor

Spherical condenser

100keV-Mott Analyzer

Laser optics equipment

20keV

4keV

○ Laser spot=φ3μm○ Electrode gap=4mm○ Voltage=20kV Field gradient=5MV/m○ Electrode: Mo (cathode)　　 material 　 Ti (anode)○ Photocathode exchanged by a load-lock system

Ceramic

Electrode

Mo

Ti

SL-PC

Dark current could be suppressedbelow 10nA under 25kV

Beam simulation

Beam simulation

adopted

20keV proto-type-gun designed for SPLEEM （ JPES － 1 ）

Mott detector system

Apparatus performanceof JPES-1

Load-locktransfer-rod

Gun assembly

Activationchamber

Apparatus Specification AchievedLaser system 　 10m spot 　　 2μm

HV Dark current 10nA

　　 6nA　(25kV)

UHV system 10-10Pa at NEA surface

　　 9×10-10 Pa

0

1

2

-3 0 3

電流密度

dI/d

X [A

/mm

]

Position [mm]

0

2

4

6

-3 0 3

電流値

Cur

rent

[A

]

Position [mm]

knife-edge

Farady cup

Gun

L

Beam sizeR=1.00±0.02mm (HWHM)

　 L=531mm

Beam size (Brightness) measurement

Source size (S) estimation= Laser spot size +Electron diffusion length= 0.65μm(HWHM) + 1μm ～ 1.5±0.3μm

Conditions ：　 Beam energy (U) = 20k ｅ V ， Beam current (I) ＝ 5.3μA

ReducedBrightness

Br I1π S2

L2

π R S 2

1

U

Current (I)

=1.0±0.4×107 A m-2 sr-1 V-1

Current density (dI/dx)

Performance of GaAs-GaAsP superlattice (Reflection PC by Nagoya group)

Polarization ~ 92%Q.E. ~ 0.5%

☆ 　 GaAs-GaAsP superlattice 　　　　　　　 shows the best performance !

@778nm

90% Polarization achieved（ 2007/10/26 ）

Uniformity of Polarization assured

Position dependence of Polarization

Transmission PC

Polarization improvement by change of

strain property of GaAsP buffer-layer

Pol. 65% Pol. 90%

Summary of JPES-1Performances

• Beam size at PC 1.3m (780nm laser)• Polarization 　 ≥　 90 ％　• Quantum efficiency 　　≥　 0.1 ％　• Average Current 　 　≥　 15A 　

• Brightness 　　　　　　　 ≥ 2107A/cm2/str (@20keV)• Brightness (reduced) ≥ 1107A/m2/str/V • NEA lifetime ≥ 200h (without beam)• NEA lifetime ≥ 30h (with 5 A)• Vacuum at PC 9.0 10-10 Pa

Performances of 20keV polarized electron gunwith transmission type photocathode (PC)

Documents on a transmission PC PES

［ Published Papers ］（ 1 ）“High brightness and high polarization electron source using 　　　　　 transmission photocathode with GaAs-GaAsP superlattice layers“　 N. Yamamoto et al. Journal of Applied Physics vol.103, (2008), 064905 （ 2 ）　“Super-high brightness and high spin-polarization photocathode”X. Jin et al. Applied Physics Express Vol. 1 (2008), Article No.: 045002［ Doctor Thesis ］　 Naoto Yamamoto ：　“ NEA-GaAs 型超格子薄膜結晶を用 い た高輝度・高スピン偏極度・大電流密度ビームを生成する電子源の開発”　　　　（ Nagoya University 、 2007 年度）［ Patents ］

1) T. Nakanishi ：　“スピン偏極電子源装置”、特願　 2006-084303 2) T. Ujihara 、 T. Nakanishi 　他 5 名：“透過光吸収フォトカソード型偏極

電子源”、特願 2008-079292 （ 2008/3/25 出願）

talk contents

1. Procedure for Higher Brightness Transmission Type Photocathode

2. A 20keV Test-Gun Apparatus Performances

Beam Performances

3. A 20keV gun for SPLEEMAssembling finished and

final beam test in progress

JPES － 2 (gun, spin-manipulator & beam SW line) for SPLEEM

High Brightness & High PolarizationElectron Source for LEEM

LEEM (Osaka)PES (Nagoya)

Within one month, this PES system will be transferred to Osaka and jointed with LEEM

Additional remarks (1)

○ Advantages of transmission-PC PES

Freedom to design both of laser & electron beam

Lines independently. Laser beam line can be optimized to satisfy various requirements.

• Minimum laser spot size obtained (this work)• Symmetrical beam distribution to beam axis• Relax the laser heating problem for ERL-PC• Two photon excitation becomes easily. • Others, etc. etc. …..

Possible applications of the new-type PES

［ SPLEEM ］Surface magnetic

domain

Magnetic memories

[TEM ］Bulk magnetic

properties

Electron holography

［ Biology ］

Chiralitystudies

［ HE Accelerators ］

High current + low emittance electron source

［ Inverse Photo-emissionSpectroscopy ］

Spin IPES

We start to contact with various fields researchers (Looking for the academic users of our PES)

SPLEEM collaboration

T. Nakanishi, S. Okumi, M. Yamamoto, [M. Kuwahara],[N. Yamamoto], [A. Mano], Y. Nakagawa

(Faculty of Science, Nagoya University)

Y. Takeda, T. Ujihara, X. J. Kim 　(Faculty of Engineer, Nagoya University)

T. Saka(Daido Institute of Technology)

T. Kato(Daido Steel Co. Ltd.)

T. Koshikawa, T. Yasue, M. Suzuki (Osaka Electro-Communication University)

T. Ohshima 、 T. Kohashi　 (Central Research Laboratory, Hitachi Ltd.)

High EnergyPhysics

SemiconductorPhysics

LEEM Physics

Electron Microscope Physics

Thanks for your attentions !

Examples of Examples of SPLEEM imageSPLEEM image

φφ

θθ

MP

A

φθ

2 mMP

MP

//

MP

//

MP

Electron injection energy

Ei=0.7 [eV] 、 50 [sec/image]

FOV=30 [m] FOV=10 [m] FOV=6 [m]

P

M

W(110)

Co : 4 ML

Mechanisms of spin-flip depolariztion

GaAs-substrate

GaP-substrate GaAs-substrate

Crystal defects of buffer-layer carried onto SL-layer

Dislocations meet with electrons

Cracks do not meetwith electrons

Crack-like defects are favorable than dislocation-like defects

SL-layers

GaAsPbuffer-layer

Spin-flip occurs Spin-flip does not occur

GaP-substrate